An expander is a tool used in pipe bursting that forces the broken, fragmented pieces of bur pipe into the soil surrounding the pipe’s pathway. When increasing the size of the pipe's passageway, the expander can create the larger bore for the new pipe.
Rear expanders are attached to the rear of the pneumatic bursting tools. Front expanders are set behind the rear of the bursting tool.
Trenchlesspedia Explains Expander
An expander can be compared to the sleeves of a sweater. If you’ve ever put on a sweater or t-shirt that was a hair too small and then pulled your arms into the garment’s body and spread your arms and hands to stretch it, your arms act like an expander. In the case of pipe bursting, the expansion is used to push the sides of the broken pipe into the surrounding soil.